What is a PU letter?

Hey, everyone. This year summer I tried really hard to get a job as an English teacher in China. And I almost got it. However when I filed all my papers to my employer she asked me if I was gonna be needing a PU letter. I didn't know what that was so I kept trying to get in touch with Chinese embassy here in Russia so they could explain to me what was PU letter and if I really need it to get work visa. For some reason none of the embassies picked up the phone nor they responded to my emails. There were no embassies in the city I live in. So I guessed that I might need it and said to my employer to get me PU letter so I wouldn't be sent back from China. She said that she couldn't get me the letter. Can someone please tell me what is a PU letter and if there are any companies who would hire non-native English speaker teacher to work in China without asking for a PU letter (I read that it is hard to get even for emoloyers).

PS. I am not Russian and don't hold Russian citizenship. I just study here.